In certain applications, multiple antennas of different operating frequencies may be co-located in proximity to one another. For example, certain vehicles, such as various automobiles, include multiple antennas of different operating frequencies within a small housing placed on the vehicle roof. Such antennas may include, by way of example, one or more monopole antennas (such as a Cell or PCS antenna) and one or more patch antennas (for example, for use with a global positioning system (GPS) device or satellite radio for the vehicle). However, the placement of such different antennas in close proximity to one another in a small housing, such as on the roof of a vehicle, may produce undesired secondary radiation from one or more of the antennas.
Accordingly, it is desirable to provide an improved antenna system, for example that reduces secondary radiation between multiple antennas having different operating frequencies, such as on the roof of a vehicle. It is also desirable to provide an improved filter for an antenna system, for example that reduces secondary radiation between antenna pairs having different operating frequencies, such as on the roof of a vehicle. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.